Pulse radar systems are capable of detecting remote targets and measuring the position (e.g. range), the radar cross section (i.e. size) and the velocity of the detected targets. When pulsed signals are used, the time period corresponding to the round trip travel of the pulse can be used to calculate target range. When pulses having relatively long pulse durations are employed, it is often difficult to detect and accurately calculate the range of two or more closely spaced targets. Specifically, with long pulses, the scattered returns from closely spaced targets overlap, preventing the return signals from being properly distinguished.
Short pulses, on the other hand, can be used to resolve closely spaced targets. However, with the use of short pulses, pulse energy becomes a consideration. Indeed, all other things being equal, a short pulse has less energy than a long pulse. When pulses having insufficient energy are used, the return signals produced have a correspondingly low energy, and cannot be detected. Since radar systems are limited in terms of peak power, it is difficult to produce a short pulse having sufficient energy to detect relatively small targets.
Pulse compression is a technique that can be used to reduce the duration of a pulse while maintaining a relatively large pulse energy. Typically, modern pulse compression techniques introduce a wideband, coded modulation into the pulse. Examples of this wideband modulation include linear frequency modulation and pseudo-random phase modulation.
When a coded pulse encounters a target, a scattered signal containing the code (or a variation thereof) is created. This scattered signal is then received and processed to “find” the code within the scattered return signal data. For this purpose, the correlation property of the code can be used. More specifically, a correlation function defined by
      r    ⁡          (      k      )        =            ∑              l        =        1            N        ⁢                  c        ⁡                  (                      k            -            l                    )                    ⁢              c        ⁡                  (          l          )                    can be used to find a so-called “zero offset” between the code and the correlation function. The location of this “zero offset” results in a peak when pulse power (usually measured in db) is plotted against range. This peak is indicative of the target range. Unfortunately, during this process, so-called “time-sidelobes” are created and show up, together with a peak, in the pulse-compressed signal. Oftentimes, the time-sidelobes of a relatively large target's return signal mask the peak of a relatively small target's signal return. In the absence of a suitable technique to overcome this problem, small targets that are in close proximity of a large target may be undetectable.
In light of the above, it is an object of the present invention to provide radar systems and methods suitable for the purposes of detecting a plurality of closely spaced targets of differing radar cross section. It is another object of the present invention to provide radar systems and methods for detecting a relatively small target having a return signal that is masked by the time-sidelobe of a relatively large target's return signal. Yet another object of the present invention is to provide radar systems and methods for detecting targets which are easy to use, relatively simple to implement, and comparatively cost effective.